brake booster rebuilt, now brakes are WAY TOO sensitive!

[MoPar~Man]

@darth_linux this is your booster:

Power Brake Booster - 54-73500

CARDONE Reman,Power Brake Booster,Chrysler 300 1970-69, New Yorker 1970-69, Newport 1970-69, Town & Country 1970-69; Dodge Monaco 1970-69, Polara 1970-69; Plymouth Fury 1970-69, Fury I 1970-69, Fury II 1970-69, Fury III 1970-69

www.cardone.com

Cardone 54-73500. Single diaphram (8.6 inch diam).

RockAuto lists applications as being 1969 - 1970 only, and only for the C-bodies (all C-bodies listed). No idea who the OEM was.

 

[Lefty71]

You say original parts are used, but wasnt your car built with the single pot master?? Your pix show otherwise.

 

[CBODY67]

How about letting the "oil pressure" return to hot base idle levels.

We know the car can be considered to be of "undetermined prior history", as the current owner is not the person who bought it new. Possibly nor was the prior owner the one who bought it new?

I'm thinking the Brake System has some digits on the Data Plate? That would answer some questions, if that designation is on the Data Plate.

Whether the booster is "correct for the vehicle" might well NOT affect the fact the pedal is not returning FULLY when the brakes are released! I'm suspecting the length of the pedal push rod was not altered when the booster was removed? Although its length and threads behind the adjusting nut could have been imaged or noted. Similar with the master cyl's pushrod.

The amount of boost available is a function of the diaphram's area, not the brand of booster. If an aftermarket booster from Summit can work decently well, so can an incorrect OEM-spec booster, FWIW.

Which gets back to the observation that the pedal does not fully return to "OFF" when the driver's foot is removed from the pedal after stopping and resuming driving forward.

Everybody have a nice holiday today!
CBODY67

 

[darth_linux]

I doubt that is a bendix booster.

This is a diagram of the Bendix booster from the 1967 Monaco / Polara manual:

View attachment 733208

Note how it is paired with an MBC with a short (not tall) reservoir. This is described as a single-diaphram booster, used with 4-wheel drum brakes. This might be an actual photo:

View attachment 733209

This booster is described as follows:

"The Bendix Power Brake Unit can be identified by the twist lock method of attaching the housing and cover together".

The next section of the manual describes the dual-diaphram (tandem-diaphram) bendix, it gives a side-profile internal view, so the external appearance can't be well visualized, it appears to have the same general shape of the single-diaphram bendix. It says:

"The Bendix power brake unit can be identified by the crimped edge method of attaching the housing and cover together."

I believe the photo (above) shows the crimped-housing dual-diaphram Bendix.

It actually says “Bendix” on the back of the housing. It’s a SINGLE diaphragm 9” Bendix. Showed up in late ‘66 and officially in ‘67.

 

[darth_linux]

You say original parts are used, but wasnt your car built with the single pot master?? Your pix show otherwise.

Yes my car had a single pot MC when I bought the car. I changed it to the '67 style dual pot MC. The booster is original to the car.

 

[darth_linux]

@darth_linux this is your booster:

http://[URL]https://www.cardone.com/product/vacuum-power-brake-booster-54-73500

[/URL]

Cardone 54-73500. Single diaphram (8.6 inch diam).

RockAuto lists applications as being 1969 - 1970 only, and only for the C-bodies (all C-bodies listed). No idea who the OEM was.

You can also find the kit and a rebuild service at Harmon Classic Brakes. Their catalog says 67-70 C body. https://www.parts123.com/parts123/y...~S79M1ED3C776282445729z~Z5Z5Z5~Z5Z5Z50001029z

 

[darth_linux]

Folks, after a good nights sleep and letting my brain do it's thing the way it does in the night, I've deduced that the MC pushrod might need to be lengthened. I believe I read here somewhere that there should be a 1/16" gap between the MC and booster before tightening the bolts, as this gap allows the pushrod to be preloaded.

Without this gap, at full pedal return the pushrod isn't even engaged with the MC yet.

When I press the brake pedal now, I feel "things" that I didn't feel before - probably the rod engaging and beginning its travel. Then, at the end of pedal travel (where I assume I have more leverage due to the geometry?) a lot of force is being applied compared to pedal travel and effort. Letting off the pedal allows the pushrod to return, but because it isn't preloaded and in constant contact, it's not quite getting enough backpressure from the MC to "push through" to the pedal assembly and fully return it to the UP position.

Pre-loading the pushrod should, in my mind, result in immediate engagement and a more smooth application of assistance, since the boost is applied when the pedal is still giving minimal leverage at the top of its travel.

SO, given that, I'm going to lengthen the pushrod and see what happens.

Now, can we please stop arguing about what booster came on an Imperial with disc brakes?

 

[Big_John]

 

[413]

I recommend to try different pushrod lengths. If that doesn't fix it call the rebuilder and have it redone.

I had cars with the super touchy pedal and it's not a pleasure to drive like that. You just paid for a rebuild, have him get it right.

I don't think it's the master cylinders job to make the brake pedal return on a power brake car.

 

[darth_linux]

View attachment 733233

I looked in the 1970 FSM where they specifically have the 9" Bendix Single Diaphragm booster like I have, and the only instruction given is to "make sure the pushrod is adjusted to the correct length". No spec is given.

I wonder if this .938 - .943 spec is specific to the Kelsey-Hayes, or applies to any/all boosters . . .

 

[Ross Wooldridge]

The pushrod should hold the master 1/16th of an inch away from booster (before snugging up the nuts) to get the correct preload on the master. Try it.

 

[Big_John]

I looked in the 1970 FSM where they specifically have the 9" Bendix Single Diaphragm booster like I have, and the only instruction given is to "make sure the pushrod is adjusted to the correct length". No spec is given.

I wonder if this .938 - .943 spec is specific to the Kelsey-Hayes, or applies to any/all boosters . . .

That isn't for the boosters. It's how far it sticks out of the booster into the master cylinder. A lot of things effect that depth, like the gasket used (or not used).

That said, it was actually taken from my '65 Plymouth FSM because I happened to have it open and it's in the section for Bendix boosters. Some of the pushrods coming out of the booster aren't adjustable like that and I don't believe the one for the '70 Chrysler is adjustable.

So, it doesn't matter what booster you have.

BTW, you should have a '66 Chrysler FSM. Service Manuals – MyMopar

If you don't want to do it that way, buy this gauge and set it to the master cylinder and then set the pushrod. It's actually a better way.

Amazon.com: BARTOO Brake Adjustment Tool Brake Booster Push Rod Adjustment Tool Master Cylinder Push Rod Length Gauge Suit for Most Brake Boosters with Adjustable Pins : Automotive

BTW, I looked at these a couple years ago and they were a lot more $$. I was even looking to 3D print something. Looking for one today... For $12, it's crazy not to get one. I might buy one just to have! LOL! You can't get a good sandwich for $12.

 

[Ross Wooldridge]

I will clarify along with @Big_John that it's the booster's pushrod into the master that has the famous 1/16" preload.

I am not sure if the brake pedal push rod that goes into the back of the booster is adjustable. If it is, as far as I know there is no preload.

 

[MoPar~Man]

I see the term or phrase "with Field Installed Hydro-Vac" associated with this booster. Regarding that, @68plymouth383 said this: "The field installed refers to the optional boosters available for 65-68. If a car from the factory had manual brakes the dealer ship would install a field installed power brake setup at the owners request." (see the link below for that discussion)

It's not clear to me if your booster (which maybe wasn't available pre-1969?) was the booster installed at Chrysler dealerships on manual-brake 4-wheel drum C-bodies. Or if your booster is a replacement for a "field installed" booster. I would still bet you 12 quatloos that your car did not come from the factory with that booster in 1966.

But this raises a question - was your car originally a manual-brake car (ie - no booster) ? And later someone installed that booster but didn't change the pedal linkage (from manual to power brakes) ? The manual-brake pedal linkage gives the driver a mechanical advantage over the power-brake linkage and would account for your current issue with the brakes being too touchy.

Also, I see comments relating to whether or not this booster will, or will not, interfere with steering column transmission shift linkage, likely linked to pre/post 1969 car models.

See more here:

power brake booster question

 

[darth_linux]

Nope. It’s PB from the factory. This car is a late ‘66. This booster was standard starting in ‘67 and was paired with the dual pot MC I currently run. I’ve seen other ‘66s also built in Delaware with the single Bendix booster. That it doesn’t appear in the ‘66 FSM is explained by the fact that the booster wasn’t put into use until 1967, and probably the tale end of the ‘66 run in some assembly lines.

I’m almost sure there is no preload on the pushrod, so I’m gonna do that and see what happens next.

 

[darth_linux]

Hey @Sixpactogo I have the Bendix booster, which doesn't use that spacer. Thanks!

 

[darth_linux]

Hey @Sixpactogo I have the Bendix booster, which doesn't use that spacer. Thanks!

RESOLVED!

So, dumb old me assumed that the booster pushrod hadn't been changed during the rebuild. A "quick eyeball" of the pushrod length before I installed the MC was "that looks close enough."

It was not.

The pushrod was way too short. I tried following @Ross Wooldridge 's advice to get a 1/16" gap between the booster and MC but that wasn't really working right. I got the rod way too long and full engagement of the brakes was almost instant (opposite from the problem I was having.)

As I was adjusting the pushrod, I came upon a portion of the threads that were rusty. Obviously, the rust showed where the pushrod was either exposed to the elements or not. I adjusted the pushrod right to the edge of the rusty threads and voila, the brakes performed just like they used to (only without the hissing from the bad booster).

Sometimes it's something like a witness mark or rust line that gives you the insight you need to put things back to "the way they were."

Cheers to all for your input, suggestions, and advice.

 

[Ross Wooldridge]

Awesome! Glad you've got the issue solved. That's why they're adjustible!

 

[Lefty71]

Sometimes it's something like a witness mark or rust line that gives you the insight you need to put things back to "the way they were."

Curious....i would have thought that the adjustment had never been changed in the first place. Were you messing with that adjustment before the booster was sent out??

 

[Ross Wooldridge]

It's actually quite easily done, and was likely done at the rebuilders.